This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/2.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract

Background

Systemic administration of chemotherapeutic agents, in addition to its anti-tumor
benefits, results in indiscriminate drug distribution and severe toxicity. This shortcoming
may be overcome by targeted drug-carrying platforms that ferry the drug to the tumor
site while limiting exposure to non-target tissues and organs.

Results

We present a new form of targeted anti-cancer therapy in the form of targeted drug-carrying
phage nanoparticles. Our approach is based on genetically-modified and chemically
manipulated filamentous bacteriophages. The genetic manipulation endows the phages
with the ability to display a host-specificity-conferring ligand. The phages are loaded
with a large payload of a cytotoxic drug by chemical conjugation. In the presented
examples we used anti ErbB2 and anti ERGR antibodies as targeting moieties, the drug
hygromycin conjugated to the phages by a covalent amide bond, or the drug doxorubicin
conjugated to genetically-engineered cathepsin-B sites on the phage coat. We show
that targeting of phage nanomedicines via specific antibodies to receptors on cancer
cell membranes results in endocytosis, intracellular degradation, and drug release,
resulting in growth inhibition of the target cells in vitro with a potentiation factor of >1000 over the corresponding free drugs.

Conclusion

The results of the proof-of concept study presented here reveal important features
regarding the potential of filamentous phages to serve as drug-delivery platform,
on the affect of drug solubility or hydrophobicity on the target specificity of the
platform and on the effect of drug release mechanism on the potency of the platform.
These results define targeted drug-carrying filamentous phage nanoparticles as a unique
type of antibody-drug conjugates.

Background

Since the introduction of monoclonal antibodies (mAbs), and the initial clinical trials
of antibody therapy in cancer patients, there has been progress in antibody based
therapeutics, particularly in oncology. The usage of naked monoclonal antibodies has
gradually evolved into drug immunoconjugates. In general drug immunoconjugates are
composed of targeting entities (mainly mAbs) chemically conjugated to a cytotoxic
drug. The outcome is improved drug efficacy with reduced systemic toxicity. To date,
the most clinically-advanced forms of armed antibodies are antibody-isotope and antibody-drug
conjugates [1-3]. Key issues in designing and testing immunoconjugates include: 1. the nature of the
target molecule, its abundance at the target, whether it is internalizing and at what
rate, and its specificity to the target, cells or tissues. 2. the linkers used to
attach the drug to the targeting moiety [4]. 3. the drug carrying capacity of the carrier is also a key issue in its potency,
thus conjugation schemes, such as the use of branched linkers were devised to maximize
the drug payload per target site [5].

A second class of targeted drug delivery platforms are the drug-carrying nanomedicines,
such as liposomes, nanoparticles, drug-loaded polymers and dendrimers [6-10]. With a few exception such as targeted liposomes, and antibody-targeted polymeric
carriers [11-14], nanomedicines do not utilize a targeting moiety to gain target specificity. Rather,
they rely on the "enhanced permeability and retention" (EPR) effect that results from
the rapid deployment of blood vessels within rapidly growing tumors resulting in blood
vessels in the tumor being irregular in shape, dilated, leaky or defective. As a result,
large drug-carrying platforms may gain selective access to the tumor while their exit
at non-target sites is limited [10,15,16]. While the immunoconjugates are limited in drug-carrying capacity, usually less than
10 drug molecules per targeting moiety [17], nanomedicines by nature deliver a much larger payload to the target cells. Recently,
a novel approach for combining antibody-mediated targeting to cell-surface receptors
with a large drug-carrying payload was provided in the description of minicells; enucleated
bacteria that are loaded with cytotoxic drugs and targeted using bi-specific antibodies
[18].

Filamentous bacteriophages (phages) are the workhorse of antibody engineering and
are gaining increasing importance in nanobiotechnology [19-23]. Phage-mediated gene delivery into mammalian cells was developed following studies
that identified "internalizing phages" from libraries of phage-displayed antibodies
or peptides. [24-31]. Recently, an efficient integrated phage/virus system was developed where tumor targeting
and molecular-genetic imaging were merged into an integrated platform [32,33].

Recently we exploited the potential of phages for targeted delivery by applying them
as anti bacterial nanomedicines. The phages were genetically engineered to display
a target-cell specificity-conferring molecule, up to 5 targeting molecules/phage if
displayed on all copies of the phage g3p coat protein. The targeted phages were chemically
conjugated, via a cleavable bond to a large payload of an antibiotic, with a maximal
loading capacity of more than 10,000 drug molecules/phage [34,35]. The anti-bacterial system was based on drug release at (and not within) the target
site. Here we present an evaluation of targeted phage nanomedicines to be applied
against cancer cells, with target-mediated internalization followed by intracellular
drug release. We show that the growth of target cells can be specifically inhibited
when the drug is conjugated either be a covalent bond or through an engineered cathepsin-B
cleavage site to the phage coat. Due to the modular nature of the platform, this new
class of targeted, drug-carrying viral particles may enable a wide range of applications
in biology and medicine.

Results

The comparative binding analysis was done by whole-cell ELISA as described in Materials
and Methods. In order to asses the affect the binding abilities of the different targeting
moieties the fUSE5-ZZ, phage vector that polyvalently displays the ZZ domain which
enables the phages to form a stable complex with target-specific antibodies was used.
The results of this assay are shown in Fig. 1. As shown, the antibody-complexed phages exhibited cell specific binding which corresponded
to the level of ErbB2 expression on the target cells, a weak binding to the control
MDA-MB231 cell line (that expresses ErbB2 at a low level) was apparent and a much
higher binding signal with ErbB2 overexpressing SKBR3 cell line indicating specific
binding. No significant signal was obtained when control phages fUSE5-ZZ complexed
with normal human IgG were used.

Figure 1.Comparative binding analysis of antibody displaying phages using whole cell ELISA. fUSE5-ZZ-chFRP5, fUSE5-ZZ-trastuzumab and control phages fUSE5-ZZ-human IgG were
added into wells that contain ErbB2 over-expressing SKBR3 cell line (black bars) or
human mammary carcinoma MDA-MB231 cells as control cell line (gray bars) that express
a low level of ErbB2. Binding was evaluated by the addition of HRP-conjugated rabbit
anti M13 antibodies followed by addition chromogenic HRP substrate TMB. The results
were plotted as absorbance at 450 nm. Data represent mean values ± SD of quadruplicate
phage samples taking from one of three independent experiments.

Evaluation of phage internalization using confocal microscopy

To function as a targeted drug carrying platform, the antibody complexed phages should
be efficiently internalized into the target cells. To address this issue, we analyzed
the capability of fUSE5-ZZ-chFRP5 complex to internalize into ErbB2 overexpressing
human breast adenocarcinoma SKBR3 cells and human epidermoid carcinoma cell line A431
cells using confocal microscopy. A positive internalization signal is typically characterized
by bright fluorescence vesicles within the cell cytoplasm together with a decrease
in membranous fluorescence. As shown in Fig. 2A, antibody-complexed fUSE5-ZZ-chFRP5 phages where internalized into both types of
cells. The orange colour dots that appeared within the cells are possibly the outcome
of a combination between the red dyed membrane and the phage marked with green colour
generated by the secondary FITC conjugated antibody, which may suggest a lysosomal
incorporation of the internalized phages.

Figure 2.Evaluation of phage internalization into A431 and SKBR3 cells using confocal microscopy. A. Immunofluorescence Staining: (a) control fUSE5-ZZ-human IgG phages (b), no phage, only antibodies (c-l) serial cuts of A431 cells that were treated with fUSE5-ZZ-chFRP5 phages. Membranes
were labelled by using red fluorescent CM-Dil cell tracker (MoBiTec, Göttingen, Germany).
Phages were detected by monoclonal mouse anti-M13 followed by incubation with FITC
conjugated goat anti-mouse IgG (green fluorescence). B. Evaluation of internalization of FITC labelled, drug conjugated phages into SKBR3
cells using confocal microscopy. a and b fUSE5-ZZ-chFRP5 phages, c and d control fUSE5-ZZ complexed with normal human IgG. As a reference, actin filaments
with the cells were labelled by using red fluorescent dye Phalloidin (Sigma, Israel).
Phages were labelled directly by FITC. Phages were added to the cells for 2 hr before
analysis by confocal microscopy. Panels a and c show the green fluorescence of Fluorescein
while in panels b and d the green fluorescence is overlaid on the red fluorescence.

Evaluation of internalization of drug conjugated phage using confocal microscopy

The internalization of antibody complexed phages following drug conjugation was evaluated
also following drug conjugation (described below). In this experiment, hygromycin
conjugated fUSE5-ZZ complexed with chFRP5 was tested with SKBR3 cells. As opposed
to detection with anti phage antibodies as shown in Fig. 2A, here the visualization of hygromycin carrying phages was obtained by conjugating
FITC to the phage through a free primary amine found in hygromycin, resulting in green
fluorescent labelled fUSE5-ZZ-chFRP5 as previously described [34]. The assay was done in different time points: 2, 12, 24 h, in order to evaluate phage
internalization rate. As shown in Fig. 2B, hygromycin carrying phages were internalized into the cells, which could be observed
2 h after adding phages to the cells. In fact, internalization seemed to be maximal
when observed at 2 h, and the fluorescent signal diminished during later time point
(not shown). In contrast, hygromycin conjugated phages that were complexed with human
IgG did not internalize. These results suggested that conjugating a large payload
of drug to the phage coat does not seen to inhibit internalization of the antibody-complexed
phages, probably occurring through receptor-mediated endocytosis.

Chemical conjugation of hygromycin and doxorubicin to phage nanoparticles

Conjugation of the two drugs to the phage nanoparticles was done by using EDC chemistry,
forming an amide bond between the exposed carboxyl side chains on the phage coat,
mostly the ones exposed on g8p, and a free primary amine on the drugs. The drugs we
used were hygromycin (an aminoglycoside antibiotics) or doxorubicin (an anthracycline
antibiotic). Approximately 10000 molecules of hygromycin were conjugated to each phage
by using EDC chemistry as we recently described [34]. EDC reaction causes the formation of a covalent bond between the phage major coat
protein, g8p and the drug which does not facilitates a controlled release form of
the drug at the target site. However, as shown in Fig. 2, the targeted phage nanoparticles are internalized into the cells possibly entering
the lysosomal compartment where they are susceptible to digestion by lysosomal proteases.
This led us to the assumption that lysosomal deconstruction of the phage may mediate
drug release within the cell.

To obtain controlled release of the conjugated drug, fUSE5-ZZ-(g8p)DFK phage was designed.
fUSE5-ZZ-(g8p)DFK phages display the lysosomal cysteine protease cathepsin-B cleavage
site on the phage major coat protein, g8p. In this phage almost all other carboxyl
groups on g8p which were susceptible to EDC conjugation were eliminated by site-directed
mutagenesis enabling the drug to be released mainly through cathepsin-B activity,
a single carboxyl group was left as an internal control for the in-vitro drug release
experiments. The sequence of native g8p in comparison with the g8p coat protein of
phage fUSE5-ZZ-DFK is shown in Fig. 3A and the scheme of the doxorubicin-loaded phage is shown in Fig. 3B.

Figure 3.Controlled release of doxorubicin release from drug-carrying phages. A. The sequence amino-acid sequence (single-letter code) of the g8p coat protein
of fUSE5-ZZ-(p8)DFK phages (top) and native fUSE5 (bottom). The mutated residues are
marked by black arrows. B. Drawing (not to scale) of a single fUSE5-ZZ-(p8)DFK phage;
In the phage scheme on the right, small turquoise spheres represent major coat protein
g8p monomers. Purple spheres and sticks represent the 5 copies of minor coat protein
g3p, which is fused to a three-color helix representing the IgG binding ZZ domain.
The Y shaped structure represents complexed IgG. An engineered g8p monomer is shown
on the left. The helix represents a partial structure of a single major coat protein
p8, conjugated through an amino terminal aspartate (D) of the sequence DFK carboxyl
side chains a molecule of doxorubicin (red). C. A Photograph of the cathepsin-B release
experiment tubes, on the right, doxorubicin carrying fUSE5-ZZ-(p8)DFK phages that
was incubated with cathepsin-B, followed by PEG/NaCl precipitation, a reddish soluble
D-DOX (verified by HPLC and MS in Fig. 5) is seen as well as a reddish pellet representing
the drug conjugated through the internal glutamate residue. On the left is a tube
containing fUSE5-ZZ phages that was incubated with cathepsin-B, followed by PEG/NaCl
precipitation, the transparent colorless solution indicate no drug release.

In vitro release of doxorubicin from fUSE5-ZZ(g8p)DFK phages

Following doxorubicin conjugation to the phages we evaluated drug release mediated
by the lysosomal hydrolase cathepsin-B. As shown in Fig. 3C, drug release could be observed by the red colour of the phage-free supernatant following
PEG/NaCl precipitation of cathepsin-B-treated, doxorubicin conjugated fUSE5-ZZ(g8p)DFK
phages. In contrast, no drug was released from similarly-treated, doxorubicin conjugated
fUSE5-ZZ phages (that do not carry the DFK sequence).

The red supernatant that was recovered from the cathepsin-B-treated, doxorubicin conjugated
fUSE5-ZZ(g8p)DFK phages was further analyzed by HPLC at the specific adsorption wavelength
of doxorubicin (480 nm). As shown in Fig. 4A, a specific peak corresponding to doxorubicin could be detected. Such a peak could
not be observed when fUSE5-ZZ(g8p)DFK phages that do not carry doxorubicin were analyzed
under identical conditions. In addition the MALDI-TOF MS analysis of this red supernatant
revealed a specific peak corresponding the weight of aspartic acid-doxorubicin adduct,
which is the N-terminal amino acid of the displayed DFK peptide Fig. 4B.

Figure 4.analysis of the Cathepsin-B released material. A. The crude cathepsin-B released material from re-suspended, PEG-precipitated,
cathepsin-B treated, doxorubicin-carrying fUSE5-ZZ-(p8)DFK phages was analysed using
a gradient of acetonitrile in water on a Waters HPLC machine (RP; C-18 column) following
the doxorubicin specific emission wavelength 480 nm. Cathepsin-B released material
eluted at 24–25 minutes post injection. B. The crude cathepsin-B released material
was analyzed by MALDI TOF MS. The theoretical mass of the aspartate-doxorubicin (shown in the insert) was observed
by the mass spectrometry Analysis as a major peak 656.04 (marked by arrow – corresponding
to the weight of aspartate-doxorubicin.

Evaluation of the cell cytotoxicity of hygromycin carrying fUSE5-ZZ phages complexed
with chFRP5 was done by in vitro cell-killing experiments. ErbB2-expressing SKBR3 cells were incubated for 48 h with
5 × 1011 of hygromycin carrying phages, and the relative number of viable cells in comparison
with cells grown in the absence of the phage was determined using an enzymatic MTT
assay. As shown in Fig. 5A, hygromycin carrying fUSE5-ZZ-chFRP5 phages inhibited target cell growth by 50%,
(Fig. 5A treatment a), a >1000 fold improvement in hygromycin potency (in comparison to the
free drug). No killing was observed when the cells were treated with, hygromycin conjugated
fUSE5-ZZ-phages in complex with human IgG (non-targeted) (Fig. 5A treatment b) or to targeted fUSE5-ZZ-chFRP5 that were not conjugated to the drug
(Fig. 5A treatment c). In contrast, the viability of HEK293 cells that were treated with the
same dose of hygromycin carrying phages was not affected at all (Fig. 5B).

Evaluation of the cell cytotoxicity of doxorubicin conjugated fUSE5-ZZ(p8)DFK in complex
with the anti ErbB2 antibodies trastuzumab (Herceptin®) (Fig. 5C) or anti EGFR cetuximab (Erbitux®) (Fig. 5D) was done with in vitro cell-killing experiments with A431 and SKBR3 cells. Here the drug was designed to
be released in a controlled manner. Doxorubicin was conjugated to the phage using
the EDC chemistry, through the engineered DFK tri-peptide where the drugs release
is mediated through the cathepsin-B activity in the endosomal-lysosomal compartments.
As shown in Fig. 5C, and 5D, doxorubicin carrying fUSE5-ZZ(g8p)DFK in complex with each of the targeting IgGs,
but also with the non-targeting human IgG caused efficient killing of the target cells,
in a dose-dependent manner. When doxorubicin was conjugated to fUSE5-ZZ phages in
complex with trastuzumab (without the DFK sequence), growth inhibition was minimal.

Discussion

This study presents targeted, drug carrying filamentous bacteriophages (phages) as
a drug delivery platform for targeting cancer cells. Our phages represent a modular
targeted drug-carrying platform of nanometric dimensions (particle diameter 8 nm,
length of a few hundred nm) where targeting moieties, conjugated drugs and drug release
mechanisms may be exchanged at will. Specifically, we have generated engineered phages
that carried either the drug hygromycin covalently linked to the phage coat, or the
drug doxorubicin linked through a cathepsin-B cleavable peptide that was engineered
into the major coat protein of the phage. As targeting moieties we used three IgG
antibodies; trastuzumab and chFRP5 that target ErbB2 and cetuximab that target the
EGFR. When target cells were treated with the targeted drug-carrying phages, selective
cell killing could be demonstrated with a potentiation factor of up to several thousand
over the free drug.

In our study we used EGFR and ErbB2 as targets; both are very well characterized in
the field of targeted anti cancer therapy. In fact, two of the three antibodies we
used as targeting moieties are used clinically to treat cancer patients (trastuzumab
and cetuximab). All three antibodies we used were shown before to facilitate the delivery
of cytotoxic payloads to target cells and tumor models [36-41]. In addition, antibody-displaying filamentous phages have been shown to undergo internalization
into target cells, which laid the foundation for proposing to use such "internalizing
phages" as gene delivery vehicles [26,29,32,33,42,43].

The drug carrying capacity of the platform is a key issue in its potency. With antibody-drug
conjugates, the amount of cytotoxic drug that can be conjugated to the antibody is
usually limited by the conjugation chemistry that, if pushed to the upper limit, may
reduce its capacity to bind antigen. As a result, such conjugates carry no more than
8 drug molecules per mAb [17]. Recently more elaborate drug conjugation schemes, such as the use of dendrimers
and branched linkers to increase carrying capacity were devised to maximize the drug
payload per targeting molecule that binds a target site [44-46]. Our phages carry as much as 104 drug molecules/phage which maximizes the intracellular drug load upon internalization
of the platform into the target cells.

Considering the linkers used to attach the drug to the targeting moiety, an ideal
linker should be stable in the serum and readily degraded within the intracellular
milieu. Some examples from the field of antibody-drug conjugates are acid-labile linkers
and enzyme-cleavable linkers [4]. We chose to evaluate two approaches; direct covalent conjugation of the drug to
the carrier and conjugation of the drug through an engineered cathepsin-B cleavage
site. Our results (Fig. 5) show that both approaches are viable, but the results may vary with different drugs
and/or targeting antibodies. Phages that were used to deliver covalently linked hygromycin
to ErbB2 expressing cells (chFRP5 IgG as the targeting moiety, Fig. 5A) could cause specific cell growth inhibition. When phages were used to deliver doxorubicin
to either erbB2 expressing cells (trastuzumab as the targeting moiety, Fig. 5C) or EGFR expressing cells (cetuximab as the targeting moiety, Fig. 5D), phages to which the drug was linked covalently were inefficient in inhibiting cell
growth, while phages that carried the cathepsin-B releasable drug were more efficient,
suggesting that with this particular phage-drug combination, an engineered drug release
mechanism is necessary to maximized potency. We could not link hygromycin to DFK-displaying
phages, since we found that hygromycin with a single amino-acid adduct (as is the
product of cathepsin-B release drug in our system) is inactive as a drug (data not
shown).

Early antibody-drug conjugates were comprised of a monoclonal antibody covalently
linked to several molecules of a clinically used anti-cancer drug. The linker connecting
the antibody and the drug was either non-cleavable or cleavable upon entry into the
cell. In the early development phase of antibody-drug conjugates, it was believed
that the tumor specificity of anti-cancer drugs could be improved merely by linking
these drugs directly to antibodies via amide bonds [17]. In most cases, the conjugates lacked cytotoxic potency and were less potent than
the un-conjugated drugs [47]. Only in the past few years the critical parameters for optimization have been identified
and have begun to be addressed. These include low drug potency, inefficient drug release
from the mAb and difficulties in releasing drugs in their active state [44]. On the basis of this much research has been focused on designing new linker technology.
The use of peptides which are susceptible to enzymatic cleavage, as conditionally
stable linkers for drugs to mAbs. The peptides are designed for high serum stability
and rapid enzymatic hydrolysis, once the mAb-drug conjugate is internalized into lysosomes
of target cells. For example cathepsin-B sensitive peptides. Cathepsin-B is a cysteine
protease found in all mammalian cell lysosomes. The cathepsin-B cleavable di-peptide
Phe-Lys was used for conjugating doxorubicin to BR96 mAb which were previously conjugated
through a hydrazone labile linker [48]. The resulting immunoconjugate showed levels of immunological specificity that had
been unobtainable using the corresponding hydrazone-based conjugates.

The objective of the experiments described herein was a feasibility study of applying
targeted drug delivery as an anti cancer tool. The system was designed on three key
components: 1) a targeting moiety, exemplified here by various monoclonal tumor specific
antibodies complexed via the ZZ domain [34]. 2) A high-capacity drug carrier, exemplified here by the filamentous phage, with
its 3000 copies of major coat protein, each amendable to drug conjugation. 3) A drug
linked directly or through a labile linker that is subject to controlled release,
exemplified here by hygromycin conjugate directly or by doxorubicin that was linked
through a cleavable peptide expressed on all copies of phage major coat protein. In
the case of covalently-linked hygromycin, we postulate that a partial non selective
release in the lysosomes post internalization would eventually lead to a specific
killing of target cell. Several features led us to use hygromycin as the model drug:
The first was the simplicity in which hygromycin can be conjugated to the phages through
simple EDC chemistry. Hygromycin has two primary amino groups, one for phage conjugations
and the other for drug or analyte conjugation (such as FITC as we have report previously)
[34]. Another important feature at this stage is the high drug solubility in water. With
this chemistry, a carrying capacity in excess of 104 drug molecules/phage was previously reported by us [34].

The second example was a controllable release mechanism that was genetically engineered
into the phage major coat protein g8p (p8). We mutated the N-terminus of p8 to express
a cathepsin-B cleavage peptide with the sequence of DFK [48]. Aspartate (D) was added to the sequence FK for the creation of two options for chemical
conjugation; through the α-amine or through the carboxyl side chain. In addition to
this mutation, the native aspartic side chains were mutated to non carboxyl side chains
(asp 11 (Fig. 3A) was retained since it is buried within the phage coat an inaccessible to conjugation
[49]). The native lys8 was mutated to glu7 in the newly mutated p8 and used as internal
control for drug conjugation and to maintaining balanced number of charged residues
that are important for phage solubilisation. Indeed, from Fig. 3C one may appreciate that there was partial release of doxorubicin upon cathepsin-B
treatment, since doxorubicin molecules linked to glu7 were not released. Since two
of the native carboxyl residues asp4 and asp5 were deleted it is important to note
that by this genetic modification in the structure of the major coat protein-8 we
have reduced the potential drug capacity by more then 60%.

Doxorubicin was used as a model drug; primary by two reasons; the first is its reporter
properties, fluorescence as well as specific emission in the wavelength of 480 nm.
This property was helpful for monitoring of drug release. The second is the relative
tolerance for conjugation of linkers into the single primary amine located to the
aminoglycoside ring tailored specifically for the solubilization of this highly hydrophobic
drug. Doxorubicin was conjugated to phages through EDC chemistry, resulting with reddish
solution. The releasing experiment with commercial cathepsin-B led to a complete release
of all connected doxorubicin molecules. Each DFK phage release about ~3500 doxorubicin
molecules as we measured by a specific reading at 480 nm with a reference of a calibration
curve of free doxorubicin. This results correlates with the maximal theoretical capacity
of this phages. Using HPLC analysis, we found that the release was limited to the
DFK phage only, while doxorubicin linked covalently to the wild-type phage coat was
not released. Further MALDI-TOF MS analysis showed that the released moiety was doxorubicin
with aspartate linked to it. Such adducts are common when labile linkers are used
do deliver drugs, and in the case of doxorubicin, do not seem to inactivate it. This
is similar to the released drug of the "non-cleavable" antibody-drug conjugates where
upon degradation in the lysosomal compartment, the drug remains covalently linked
to a single amino acid, either lysine for maytansinoid conjugates or cysteine for
auristatin conjugates [50-52].

Internalization of the phages, unconjugated or armed with drug could be demonstrated
by fluorescence confocal microscopy (Fig. 2). The result of the target cell killing assays showed that the soluble drug hygromycin
connected through a non cleavable stable amide bond, directly to phage coat carboxyl
residues, could achieve impressive potency improvement, in factor of >1000 over the
free drug (Fig 5A) (5 × 1010 phages carrying 104 drug molecules/phage, carry 0.43 μg free drug, which inhibits cell growth as well
as ~1 mg of free drug). This occurred although poor drug release within the cells
could be expected.

The interpretation of the results of doxorubicin-carrying phages is more complex,
since the goal of this system was a proof of new concept for the construction of cleavable
linkers by genetic engineering instead of conventional organic chemical linkage. Our
results show the DFK peptide to be specifically cleaved by cathepsin-B, specifically
at the engineered site (Fig. 3) and within the target cells. Further, doxorubicin-carrying DFK phages (Fig. 3B and 3C, white bars) are more potent in comparison the phages that carry covalently-linked
doxorubicin (Fig. 3B and 3C, grey bars) even though the latter carry 10000 drug molecules/phage while the former
carry 3500 drug molecules. Moreover, phages to which doxorubicin was covalently linked,
although they were target-specific, inhibited cell growth less efficiently than non-specific,
human IgG linked DFK phages, further demonstrating the contribution of the engineered
drug-release mechanism to the potency of the platform. As for the limited specificity
of doxorubicin-carrying DFK phages, we have already observed that coating phages with
a high density of hydrophobic molecules limits the solubility of the phages [34] and cases them to become "sticky", that is, to bind non-specifically to bacteria
and to cells (unpublished results). The results shown in Fig. 5B and 5C suggest that this may also be the case with doxorubicin-carrying phages, since comparably
levels of cell-killing could be observed when that phages were linked to the targeting
antibodies, or to the irrelevant control, normal human IgG. Such non-specific killing
could not be observed with hygromycin-armed phages (Fig 5A). Doxorubicin is known as a drug of which doses are limited by unwanted toxicity
to non-tumor tissues [53]. Doxorubicin and other anthracyclines are amphiphilic molecules known to interact
with cell membranes [54], which may cause non-specific binding. Large polymer-doxorubicin conjugates were
reported as having limited solubility [55], which may also affect target specificity. One may concluded that drug delivery platforms
that carry the drug on the outside will be limited to highly water soluble drugs that
do not bind non-discriminately to cells. However, a remedy to this limitation may
be found in linking the hydrophobic drug to the phage through a solubility-enhancing
linker, as we have recently reported [34]. We have shown that the potency and the target specificity of anti bacterial chloramphenicol-armed
phages was substantially improved when this hydrophobic drug was linked to the phage
coat through small hydrophilic molecules that served as solubility-enhancing linkers.
On the other hand, phages that were directly armed with the hydrophobic drug chloramphenicol
were less specific [34]. An additional advantage of such an added hydrophilic coat is that it reduces both
the immunogenicity and antigenicity of the drug-carrying phages upon injection into
mice (unpublished data).

Conclusion

To conclude, our study demonstrated a proof of principle of targeted, drug-carrying
filamentous bacteriophage as anti cancer agents. The issues of Pharmacokinetics, biodistribution
and immunogenicity and tumor penetration: these parameters are key issues in current
phage therapy studies [56,57]. Basically, phages are immunogenic on one hand, and upon intravenous injection are
removed quickly by the reticuloendothelial system on the other. Attempts to modulate
phage pharmacokinetics were based on isolating long-circulating mutants of phage lambda
[56]. But no such studies were done with the filamentous phages. We believe that chemical
modification of the phage coat (as we do during drug conjugation) should modulate
the pharmacokinetics, the biodistribution and the immunogenicity in comparison to
bare phages. Regarding tumor penetration, phages can not be regarded as huge complexes
as may be evident from their molecular weight which is about 15 million dalton. Due
to their needle-like structure, they may very well penetrate into tumors, as may be
suggested by the study where xenografts in nude mice were eradicated following IV
injection of phages that delivered a therapeutic gene [32]. We are currently comparing the immunogenicity, pharmacokinetics and biodistribution
of un-conjugated to drug-carrying phages in animal studies.

All the chemicals used were of analytical grade and were purchased from Sigma (Israel).
Unless stated otherwise, reactions were carried out at room temperature (about 22°C).

Linking phages to the targeting antibodies

Phage fUSE5-ZZ that can be complexed with IgGs was recently described [35]. Briefly, this is a derivative of the filamentous phage vector fUSE5 [58] that was engineered to display the IgG Fc-binding ZZ domain on all copies of the
g3p minor coat protein. Filamentous phages were routinely propagated in DH5-αF' cells
using standard phage techniques as described [59]. Phages were usually recovered from overnight 1 litre cultures of carrying bacteria.
The bacteria were removed by centrifugation and the phage-containing supernatant was
filtered through a 0.22 μm filter. The phages were precipitated by addition of 20%
(w/v) polyethylene glycol 8000 PEG/2.5 M NaCl followed by centrifugation as described
[59]. The phage pellet was suspended in sterile miliQ double-distilled water (DDW) at
a concentration of 1013 pfu/ml and stored at 4°C.

To form a complex with targeting antibodies, 1012 phage in 1 ml PBS were mixed with 1.6 μg of the IgG: chFRP5 [36],

or control normal human IgG (Sigma, Israel). The phage-IgG mixtures were left for
at least 1 h at room temperature. This phage to IgG ration yields occupancy of about
50% of the available ZZ sites on the phage [35].

Construction of phage fUSE5-ZZ-(g8p)DFK

fUSE5-ZZ was further modified to display the lysosomal cysteine protease cathepsin-B
cleavable DFK tri-peptide [48] on all copies of the g8p major coat protein. The DFK tri-peptide was inserted into
the N-terminal region of g8p by two subsequent steps. First, fUSE5-ZZ DNA was amplified
by PCR using primers P8-DFK/R-FOR (5'-CTGACTTTARGGGTCCTGCAGAAGCGGCCTTTGACTCCC-3')
with M13g3BamHI-REV (5'-TATTCACAAACGAATGGATCC 3') and in a second reaction using primers P8-DFK/R-REV
(CAGGACCCRTAAAGTCAGCGAAAGACAGCATCGGAACG-3') with P5-BsrGI-FOR (5'-TCGTCAGGGCAAGCCTTATTC-3'). Second, the resulting DNA fragments were assembled
using primers P5-BsrGI-FOR and M13g3BamHI-REV. The assembled PCR product was purified, digested with restriction enzymes
BsrGI and BamHI, and cloned into a similarly digested fUSE5-ZZ phage vector. The resulting phage
was named fUSE5-ZZ-(p8)DFK. The g8p major coat protein of fUSE5-ZZ-(p8)DFK contains
an amino terminal aspartate for drug conjugation by ECD chemistry to its carboxyl
residue followed by the cathepsin-B cleavage site phenylalanine-Lysine (FK). In addition,
it contains mutations that eliminated almost all of the naturally occurring free carboxyl
groups on g8p that may be susceptible to EDC conjugation.

Drug conjugation to phages by the EDC chemistry

The phage major coat protein g8p contains 3 carboxylic amino acid (glu2; Asp4; asp5;)
that can be conjugated by application of (1-Ethyl-3- [3-dimethylaminopropyl] carbodiimide
(EDC) chemistry, a rapid reaction performed at mild acidic pH (4.5–5.5) [60]. All the conjugations were done within a total volume of 1 ml of 0.1 M Na-citrate
buffer; pH = 5, 0.75 M NaCl, 2.5 × 10-6 mol of the aminoglycoside, 5 × 1012 and phages that were already complexed with the targeting IgG. The reaction was initiated
by the addition of 2.5 × 10-6 mol of EDC, which was repeated two more times at time intervals of 30 min. Reactions
were carried out at room temperature (~22°C) with gentle stirring in 2 ml Eppendorf
tubes for a total of 2 h. The targeted drug-carrying phage nanoparticles were separated
from the reactants by two successive dialysis steps of 16 h each against 1000 volumes
of sterile 0.3 M NaCl.

Quantifying linked Doxorubicin molecules/phage by cathepsin-B cleavage in a cell-free
system

2 × 1012 doxorubicin-conjugated fUSE5-ZZ-(g8p)DFK phages were suspended within 300 μl cathepsin-B
reaction buffer as described [48]. Next, 7 units of cathepsin-B (Sigma, Israel) were added for 24 h at 37°. The phages
were precipitated with PEG/NaCl and the supernatant was analyzed by reverse-phase
HPLC and MALDI-TOF MS. For the HPLC analysis, a reverse phase C-18 column was used
on a Waters machine with a gradient 0% to 100% of acetonitrile in the mobile phase,
at 1 ml/min flow rate. Under these conditions, the doxorubicin containing peak at
480 nm was eluted 24–25 min after sample injection.

Evaluation of target cell binding by whole-cell ELISA

Unless stated otherwise, all secondary antibodies, HRP-conjugated or fluorescent were
from Jackson Immunoresearch Laboratories (USA). Evaluation of the binding ability
of fUSE5-ZZ phages complexed with chFRP5 or trastuzumab to SKBR3 cells was done by
whole-cell ELISA [36]. Following trypsinization, cells were washed once with 2% foetal calf serum, in PBS
(incubation buffer, pH 7.4). In each experiment approximately 106 cells were divided into individual immunotubes (Nunc, Sweden). To confirm the specificity,
phages (1010 and 1011 phage/ml) were added to the cell tubes for 1.5 h at 4°C. After washing ×2 with incubation
buffer, HRP-conjugated rabbit anti M13 antibodies (GE healthcare, USA, 1/5000 dilution)
were added to the immunotubes for 1 h at 4°C. Detection of cell bound phage was performed
by addition of 0.5 ml of the chromogenic HRP substrate TMB (Dako, USA) to each tube
and colour development was terminated with 0.25 ml of 1 M H2SO4. Finally, the tubes were centrifuged for 10 min at 4000 rpm and colour intensity
of supernatants was measured at 450 nm.

Analysis of phage internalization using confocal microscopy

Internalization of IgG-complexed phages into SKBR3 and A431 cells was studied using
confocal microscopy as follows: Cells were grown on 24 mm cover slips in DMEM supplemented
with 10% FCS essentially as described [36]. Subsequently, the medium was replaced by 450 μl DMEM without FCS into which 5 ×
108 phages were added in 50 μl. After 3 h incubation at 37°C, the cells were gently washed
×3 with PBS and 100 μl of 1 μg/ml of membrane labelling CM-Dil (Molecular probes,
USA) were added following incubation for 5 min at 37°C and 15 min at 4°C. Next, the
cells were washed ×3 with PBS and fixed by 30 min incubation with 500 μl of 4% formaldehyde
followed by washing with 1 ml PBS. To ensure efficient cell permeability, cells were
washed with 250 μl of 0.2% triton × 100 in PBS, after which cells were blocked with
90% FCS in PBS containing 0.05% Tween 20 (Sigma, Israel) for 30 min. The blocking
solution was aspirated and monoclonal mouse anti-M13 (1/100 dilution; GE healthcare,
USA) was added for 1 h incubation following ×2 washes with 2% BSA in PBS. Subsequently,
cells were incubated with 1:100 diluted FITC conjugated goat anti-mouse IgG for 1
h at room temperature. Finally, the cells were gently washed ×3 with PBS and images
were acquired using a LSM 510 laser scanning confocal microscope (Vontz 3403B).

Cell viability assay

The in-vitro cell-killing activity of hygromycin carrying fUSE5-ZZ-chFRP5 or doxorubicin carrying
fUSE5-ZZ-(g8p)DFK-trastuzumab and fUSE5-ZZ-(g8p)DFK-cetuximab antibody complexed phage
nanoparticles was measured by an MTT assay. Human breast carcinoma A431, SKBR3 target
cells, or HEK293 control cells were seeded in 96-well plates at a density of 104 cells/well in DMEM supplemented with 10% FCS. |Targeted drug carrying phage nano-particles
and relevant control phages were added to samples in 100 μl containing 5 × 1011 phages and serial three-fold dilutions thereof and the cells were incubated at 37°C
in 5% CO2 atmosphere. 48 h later, the media was replaced by phage-free media (100 μl per well)
containing 5 mg/ml MTT reagent (Thiazolyl Blue Tetrazoliam Bromide, Sigma, Israel,
dissolved in PBS) and the cells were incubated for another 4 h. MTT-formazan crystals
were dissolved by the addition of 20% SDS, 50% DMF, pH 4.7 (100 μl per well) and incubation
for 16 h at 37°C in 5% CO2 atmosphere. Absorbance at 570 nm was determined on a microtiter plate reader. Identical
concentrations and combinations were tested in four separate wells per assay and the
assay was performed at least three times. The results were expressed as percentage
of living cells in comparison to the untreated controls that were processed simultaneously
using the following equation: (A570 of treated sample/A570 of untreated sample) ×100.

Authors' contributions

HB designed and carried out experiments and analyzed the data; IY designed and carried
out experiments, participated in analyzing the data and in writing the manuscript.
IB designed the study and wrote the manuscript.

Acknowledgements

This study was supported in part by a grant from the Israel Public Committee for Allocation
of Estate Funds, Ministry of Justice, Israel, and by the Israel Cancer Association.
I.Y. was supported by a Dan David Ph.D. scholarship for young scholars in future dimension.